#whatWM? e pluribus unum: Consensus despite diversity

Klaus Oberauer. Nelson Cowan's review of definitions of "working memory" provides a very useful overview of the different ways in which the term has been and is still being used in the literature, and the potential confusion that can be caused by its multi-faceted meaning.

Some of these definitions strongly reflect the historical roots of the concept, for instance the "computer WM" definition, inherited from the computer metaphor of the human mind, and the "storage and processing WM" definition, reflecting the popularity of dual-task paradigms for studying working memory. Others, such as "attention control WM" and "inclusive WM", define working memory through the correlates of working-memory tests in individual-differences studies (i.e., attention-control tasks and long-term memory tasks, respectively). Still others, such as "life-planning WM" and "long-term WM", reflect theoretical considerations about the mechanisms a cognitive system needs to fulfil some of its core functions, such as planning complex actions and providing seamless access to the vast amount of knowledge in our long-term memory.

One thing Cowan's review shows is that the definition of "working memory", like that of every scientific concept, depends on the theory in which the concept figures. The theory dependence is most obvious when a definition of "working memory" is directly tied to one theory – for instance, the "multicomponent WM" definition is the definition given in the classic theory of Baddeley and Hitch. Other definitions are more general so that they can be used in several theories, but even the most generic definition, appropriately labeled "generic WM", includes theoretical assumptions that distinguish it from some other definitions, such as the assumption that working memory is limited in how much information it can hold, and that its contents are short-lived. As long as we have competing theories of working memory, we will have different definitions of that concept. As our theories evolve – hopefully becoming more precise and more accurate – our definitions of "working memory" will change.

The theory-dependence of definitions is a sign of scientific progress (Quine, 1977). Concepts serve us insofar as they enable successful generalizations. Scientific concepts are defined to be used in scientific generalizations. For instance, we define water as "H2O" because statements such as "If a liquid consists of H2O then it turns gaseous at 100 degrees centigrade" are empirically more successful than statements such as "If a liquid is transparent and odorless and occurs plentifully in our environment, it turns gaseous at 100 degrees centigrade". Every theory of working memory aspires to make successful statements such as "If some information is in working memory, then X is true about it", where X could be, for instance, "the information can be used to control novel actions". Different theories of working memory make different assumptions about what features working memory must have so that generalizations like the above are true about it. A mechanistic theory of working memory will eventually explain why and how information in working memory enables control of novel actions, in the same way as contemporary chemistry explains how the characteristics of molecules determine the boiling point of a liquid. Then we will be able to define working memory in terms of these mechanisms.

The multiplicity of meanings of the term "working memory" is frustratingly confusing, especially when looked at from outside the field. Are researchers using the term even talking about the same thing? To a large extent, I think, they are. The various definitions are grounded through the experimental paradigms and the empirical phenomena that they are applied to. Across the diversity of theoretical approaches and the definitions that come with them, there is a remarkably strong consensus among researchers of "working memory" about which paradigms and phenomena are informative for that construct.

Some of us recently experienced that consensus during a collective effort towards collating a set of benchmark findings for working memory: Seventeen researchers on working memory with very heterogeneous theoretical perspectives came together to agree on benchmark findings, defined as findings that are robust and important for theories of working memory to explain. With a few exceptions, there was little disagreement about whether a given finding was "about" working memory. Researchers in the field appear to have a shared understanding of what needs to be explained by a theory of working memory.

This consensus is not arbitrary—it is backed by the fact that performance on most of the paradigms we use to experimentally study working memory correlates highly across individuals, so that scores from diverse paradigms reliably form a general "working memory" factor that accounts for a large proportion of variance in the individual indicators (Cowan et al., 2005; Oberauer, Süß, Schulze, Wilhelm, & Wittmann, 2000; Wilhelm, Hildebrandt, & Oberauer, 2013). This fact provides construct validity to the concept of working memory. In this regard our field is in a better situation than other research fields investigating cognitive processes with a common label. For instance, factor analytic studies have not yet provided evidence for a general factor of "visual attention" (Skogsberg et al., 2015), and indicators of "executive functions" have a habit of correlating only weakly and erratically with each other (Keye, Wilhelm, Oberauer, & van Ravenzwaaij, 2009; Krumm et al., 2009).

Looking at what is common among the experimental paradigms that load strongly on a general working-memory factor, and at what is common among the theoretical interpretations of these paradigms, I think we can give a characterization of what we are talking about when we use the term "working memory". I propose this as a working definition: "Working memory" refers to the mechanisms and processes that hold the mental representations currently most needed for an ongoing cognitive task available for processing. This working definition is not a scientific definition because it does not play a role in any theory, but it serves the purpose of communicating the scope of our research efforts. It also enables us to find within each mechanistic theory of cognition which mechanisms and processes fulfil the function of "working memory" – this might be something unitary, or a collection of mechanisms that would not be conceptualized as a single entity in that theory. My working definition is very similar to Cowan's "generic WM", but drops the theoretical assumptions that working memory has a capacity limit (because I think of the capacity limit as an empirical fact, rather than as a feature characterizing the extension of the concept of working memory) and the assumption that it is short-lived (because for all we know, information in working memory lasts as long as it is needed for the person's goals—usually not long, but that says more about the pace of our thinking than about working memory).

Taken together, for a field that is still far from a theoretical agreement, we are in a fairly good position to say what various manifestations of "working memory" have in common, both conceptually and empirically. As a way forward to reduce the confusion and miscommunication that Cowan pointed out in his review article, I suggest that we distinguish between a working definition that circumscribes our shared understanding of what "working memory" is about, and the various scientific definitions of the term in the context of competing theories.